The present invention pertains to mixing apparatus for mortar, plaster and the like and, more particularly, to a locking apparatus for preventing inadvertent or unintended rotation of the mixing drum from a mixing to a dumping or discharge position.
Small portable mortar and plaster mixers are well known in the art and typically include a cylindrical or semicylindrical-walled mixing drum supported on a frame for rotation on the drum axis between an upright mixing position and a rotationally tilted dumping position. Also typically, an engine or motor is mounted on the frame adjacent the drum for driving mixing blades or mixing paddles rotatably mounted inside, either on the aids of the drum or on a separate axis parallel to the drum axis. The mixer frame is usually mounted on wheels for trailering and support during operation where a third supporting leg on the trailer tongue provides, with the wheels, three-point support for the mixer.
The ingredients for a desired mix are dumped into the open top and, after mixing, a dumping lever attached to the drum is pulled by the operator to rotate the drum on its axis until the mix can be dumped by gravity, supplemented by continued rotation of the mixing paddles, out of the open portion of the drum. After dumping, the operator reverses the force on the lever arm and rotates the drum in a reverse direction to the original mixing position where, typically, a stop on the drum surface engages the mixer frame to prevent further reverse rotation beyond the mixing position.
In order to prevent inadvertent rotation of the drum from the mixing position, a manually releasable locking device is often mounted on the frame from which it extends into locking engagement with the drum. One very common type of prior art locking mechanism comprises a spring loaded pin mounted on a vertical frame member adjacent one flat end wall of the drum and which is biased into engagement with a stop on the drum wall. To unlock the pin, the operator pulls against the bias of the spring, generally in a direction parallel to the axis of drum rotation. However, the operator must also simultaneously engage the dumping lever with the other hand and begin to tilt the drum for discharge before the spring biased locking mechanism is released. The two separate motions required of the operator are in perpendicular directions, making it difficult for the operator to tilt the drum. The force required to tilt the drum from the mixing to the discharge position is compounded by rotation of the mixing paddles which place a torque on the drum in the opposite reverse rotational direction. As a result, it is not uncommon for the operator to be required to use both arms to engage the tilt lever arm and rotate the drum. Therefore, another person may be required to unlock the locking device. Worse yet, to avoid the use of another workman, there may be the temptation to disenable the locking device entirely.
Therefore, a simple, yet effective mixer drum locking apparatus that can be operated by one person would be most desirable. The locking device should enable the operator, if necessary, to grasp the drum tilt lever with two hands for tilting discharge. Furthermore, the locking device should be automatically operable to relock the drum when it is returned to the mixing position.
In accordance with the present invention, a locking arm is pivotally attached by one end to the mixer drum for rotation on a pivot axis that is generally parallel to the rotational axis of the drum. When the drum is in the generally upright mixing position, the arm is pivotable between a locked position where the opposite end of the arm is in locking engagement with the frame to an unlocked position where the arm is rotationally displaced from the locking position. The arm is further pivotable from the rotationally displaced unlocked position back to the locked position in response to reciprocal drum rotation in a first direction from the mixing position to the dumping position and reverse rotation back to the mixing position. The drum may also include a stop device that is engageable with the frame to prevent reverse rotation of the drum past the mixing position.
In the preferred embodiment, the frame extends under the mixer drum and the locking arm is mounted to rotate freely on its pivot axis and, when in the locked position, the pivot axis is positioned up from the frame with respect to drum rotation in the first direction. The opposite end of the locking arm includes a locking tooth that engages the fame in the locked position and prevents arm rotation in one direction. The locking arm also includes an abutment surface positioned adjacent the locking tooth and adapted to engage the frame in the locked position. Preferably, when in the locked position, the locking arm extends angularly downwardly from the pivot and the locking tooth rests by gravity on the frame.
In the preferred embodiment, the mixer drum includes a pair of opposite end walls that enclose the cylindrical outer wall and a mixing device mounted for rotation in the drum. The frame includes a generally horizontal frame member positioned under the drum and extending generally parallel to the drum axis. The locking arm is pivotally attached to one of the end walls. In the preferred arrangement, the locking arm includes an abutment surface on the opposite end which is positioned to engage the frame member in the locked position and a locking tooth on the end of the arm positioned above the abutment surface and adapted to rest by gravity on the frame member in the locked position. A stop device is mounted on the drum and is engageable with the frame to prevent reverse rotation of the drum past the mixing position. Preferably, the locking tooth engages a frame member surface along a tooth flank of a given length, and the stop device is positioned to prevent reverse rotation of the drum from the locked position by a distance greater than the length of the tooth flank.